Server-based computing allows a networked client device, remotely situated with respect to a server computing system, to access computing resources on the server. For example, the client device may run desktop client software and hardware that uses a remote desktop protocol, such as Remote Desktop Protocol (RDP), Virtual Network Computing (VNC), or Personal Computer over Internet Protocol (PCoIP), to access a desktop remotely. The desktop client software displays an image of a graphical user interface (GUI) generated by the operating system and applications running at the server computing system. For example, a virtual machine running on the server computer system transmits to the networked client device a continuous flow of GUI images generated by an operating system running on the virtual machine, and applications running on the operating system. The virtual machine employs a remote desktop protocol and a virtual network interface card (VNIC) to communicate with the client device. Using the same VNIC, the virtual machine also allows applications running in the user space of the operating system to interact with other network destinations. However, allowing all applications running in the user space of the virtual machine's operating system to operate and access the same VNIC makes the system vulnerable to misuse.